#include<stdio.h>
unsigned long f(int n);
int main(void){
    int n;
    scanf("%d",&n);
    printf("%lu",f(n-1));
    return 0;
}
unsigned long f(int n){
    unsigned long num[90]={1,1};
    int i=2;
    for(i=2;i<=n;i++){
        num[i] = num[i-1]+num[i-2];
    }
    return num[n];
}